Phead I personally think your isolating your threat to "just Apple" approach is too simplistic. Apple is a massive company with tons of employees and teams. On the surface, it looks like they all maintain the same standards and play well together, but that's not guaranteed. It's always possible for something to go wrong, whether accidentally or maliciously. Apple can (and has) been breached before. And at that point, if you went full Apple ecosystem, an attacker just potentially got everything instead of a subset of data that you singled out to one of multiple third-party services. Your security for all of your data is now diminished to the security of Apple's weakest link.
Also, since it's not open-source, you really don't know who else has access to the data outside of Apple and its impossible to know if they ever actually delete anything even when you ask them to. Apple is also free to change its terms whenever it pleases and at which point, all you'll have is vendor lock-in without any of the perceived benefits.
I think even if someone were to use iOS, they would still benefit from using some third-party apps. Just as an example, email, contacts, and calendar aren't encrypted, even according to Apple, which defeats some of the perceived privacy perks of iCloud+. So at the very least, using something like Proton/Tuta/etc would still be beneficial even on an iPhone.